The problem you will face is that you have no way to verify that you haven't broken something unrelated when you make a change, because the current behavior of the system is unknowable; you can't write comprehensive tests for a codebase that large and that bad because you don't even know what it's supposed to be doing. The chickenshit nature of PHP and the lack of a sensible type system and refactoring tools will make things even more difficult.

So, run. Seriously. You're doomed.